This invention relates to vibration stabilizers and more particularly to apparatus for reducing the vibration of a tube within a tube bundle in a shell and tube heat exchanger.
A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tube sheet forming a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The area below the tube sheet formed by the inlet and outlet plenums is the channel head where some of the repair and maintenance work on the steam generator is performed.
The primary fluid having been heated by circulation through the nuclear reactor core enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, the primary fluid flows upwardly through first openings in the U-tubes near the tube sheet which supports the tubes, through the U-tube curvature, downward through the second openings in the U-tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid known as feed water, is circulated around the U-tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tube into the secondary fluid surrounding the tubes causing a portion of the secondary fluid to be converted to steam. The primary fluid generally flows at a velocity which is low enough such that little or no vibration of the tubes results therefrom; however, the velocity of the secondary fluid or feed water is such to induce unwanted vibration into the tubes resulting in problems hereafter discussed.
Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-tube walls and the tube sheet, it is important that the U-tubes and the tube sheet be maintained defect-free so that no breaks will occur in the U-tubes or in the welds between the U-tubes and the tube sheet thus preventing contamination of the secondary fluid by the primary fluid.
Although in some steam generators, the tubes are not U-shaped but straight, both types of tube bundles in steam generators are subjected to the above set forth flow induced vibration due to the velocity of the secondary fluid or feed water across the tube bundle. Therefore, when tubes in the tube bundle degrade due to corrosion and wear and perhaps due to the flow induced vibration, it is necessary not only to plug the degraded tubes in order to prevent contamination of the secondary fluid by the primary fluid but also to control or reduce the vibration in the degraded tubes to prevent the degraded tubes from coming in contact with the adjacent usable tubes leading to unexpected degradation of additional tubes.
Prior attempts to diminish the vibration of degraded tubes involved the use of rigid sleeves or rods positioned inside the degraded tubes or in some cases barstock connected to the tube. In general, it has been believed that if the natural frequency of the tube can be raised above the frequency of the induced vibration produced by the feed water, then the vibration of the degraded tube would lessen. Since stiffening the tube raises the natural frequency of the tube but adding mass to the tube lowers the natural frequency of the tube, it has always been thought that the most efficacious method to reduce vibration was one which added the maximum amount of stiffness with the least amount of mass. Accordingly, rigid sleeves or rods have been used heretofore.
Because the channel head is a relatively small area and the place from which rods, sleeves or the like must be inserted, prior devices have required installation of the inserts in small sections by threading individual sections of tubes or rods in the channel head in order to construct an insert of desired length and then finally welding the end of the insert to the tube end to seal same. Such a construction could take as long as ten minutes per tube and in the case of a tube bundle having 7,000 tubes of which several hundred may have to be plugged, it can be seen that a significant amount of radiation exposure to maintenance personnel resulted from prior art methods and apparatus for reducing tube vibration.
What is needed is a vibration stabilizer for degraded tubes within a tube bundle in a shell and tube heat exchanger which is effective to reduce vibration of degraded tubes but at the same time is installed quickly, thereby reducing radiation exposure to maintenance personnel.